Semi-inclusive processes form the backbone of much of the physics program at the future EIC. They will provide crucial information on fragmentation and the hadronization mechanism and also serve as powerful probes of hadronic structure. They also test key concepts of the theoretical analysis of QCD at high energies, among them factorization, universality, and perturbative calculations. Given the importance of semi-inclusive physics at the EIC, improving the precision of the theoretical description is one of the central tasks for EIC theory. Project P4 will address a suite of entirely new aspects of semi-inclusive deep-inelastic scattering (SIDIS) processes at the EIC. An overall trait of our proposed studies is that they combine high-order precision computations in QCD perturbation theory with phenomenology for the future EIC. Specifically, we plan to address the following topics: 1) SIDIS at high perturbative orders: Here we plan to extend our next-to-next-to-leading order QCD calculations for SIDIS of the first funding period of FOR2926 to electroweak SIDIS, which is important for exploring the rich spin structure of nucleons. We also plan to extend previous studies of all-order QCD resummation for SIDIS by including terms that are sub-leading near partonic threshold. 2) Precision hadronic structure from semi-inclusive reactions: This subproject addressesnew types of SIDIS observables at the EIC, hadron pair production, spin transferphenomena, and photoproduction reactions. For each of these, we plan to compute higher-order QCD corrections, either at fixed next-to-leading order of QCD, or withthreshold resummation. 3) Development and application of modern event generators for lepton-hadron colliders:This part of the project builds on the most successful phenomenological studies on transverse-momentum dependent parton distributions for SIDIS carried out in the first funding period, especially the impact studies for EIC presented there. Building connections to the physics of parton showers is the natural next step toward precision phenomenology at the EIC. We also anticipate much synergy between the resummation part of our project and project, thanks to the close relations that exist between parton showers on the one hand and all-order QCD resummation forhard-scattering reactions on the other.
